There were many variables that impacted the communication in this film. Some overlapped others and were difficult to distinguish. Three of the more obvious communication variables were age, prejudice, and self-concept. Below I've listed some examples from the film and their impact as I saw it.
AGE.
At first the old man is given deference due to his age. For example, near the beginning of the film, the painter pulls the chair out for him to sit. Later, when the loud-mouth verbally attacks the old man, the painter quickly jumps to his defense, stating, "you shouldn't speak to an old man that way."".
He probably doesn't realize that labeling him as an "old man- and singling him out for preferential treatment due to his age, probably diminishes the old man's credibility with the group as a whole.
The term "kid- is frequently used throughout the film as a derogatory slur aimed at various members of the jury to point out their behavior as immature or churlish. The accused is also referred to as a "kid-. This automatically associates him with these undesirable traits on a sub-conscious level.
Age again comes into play when the old man is able to empathize with the witness also labeled as an old man. His similarity in age with the witness lends credibility to his argument that the old man may have been seeking attention and therefore lied or exaggerated when asked what he saw or heard.
PREJUDICE.
The ill man quickly shows his prejudice when the defendant's alibi is brought up. He informs the other jurors that he's lived among "them- all his life and you can't believe a word they say because they're born liars. His argument is quickly shot down when he is asked to explain why he believes the old lady when she is also, "one of them-. This causes irreparable damage to his believability by the non-prejudice jurors. .
The loud mouth automatically assumes that Jack Klugman is the one who first changes his vote because he finds it easy to dislike him because of his background.